In a mobile communication network for mobile telephones or the like, cells managed by radio base stations have various sizes. These cells are named according to the radiuses of cells; namely, cells not smaller than 1 km in radius are referred to as macro cells, cells about 0.5 to 1.5 km in radius, pico cells, and cells about 10 to 500 m in radius, femto cells. Of them, femto and pico cells are also referred to as small cells, and hereinafter the term “small cell” will be used unless otherwise noted.
The setup of a small cell makes it possible to achieve coverage compensation and capacity increase. Coverage compensation is required to cover an area where radio waves do not reach because of buildings and the like within a macro cell. For example, sufficient mobile communication services can be provided by placing a small cell base station inside a building where radio waves from a macro cell base station do not reach due to penetration losses caused by building walls and the like. Moreover, capacity increase is required to handle mobile traffic rapidly increasing due to the popularization of mobile phones, particularly, smart phones. With small cells, the number of users that can be accommodated per cell is lessened by shortening the cell radius, whereby the overall capacity can be increased.
As described above, the deployment of small cells makes it possible to achieve coverage compensation and capacity increase. However, in a system where macro and small cells coexist, a problem arises that the probability of radio wave interference increases, leading to degradation in communication quality. To address such a problem, heterogeneous networks employ such a configuration that small cells are entrusted with hot spots and radio wave-unreachable areas while macro cells cover other areas, with appropriate interference control, power control and the like being performed.
Further, as to cell reselection control in mobile communication networks, NPLs 1 to 4 describe technologies for control at the times of cell reselection and handover. NPLs 1 and 2 define T_reselection, which is a cell reselection timer value. NPL 3 defines 31 seconds as the maximum value of the T_reselection timer in 3G networks and NPL 4 defines 7 seconds as the maximum value of the T_reselection timer in LTE networks. Accordingly, in LTE networks, if the T_reselection timer is set to a maximum value of 7 seconds, cell reselection determination can be performed by measuring radio wave quality in another cell during 7 seconds, and if radio wave quality in the other cell is good, cell reselection can be executed. Furthermore, if Speed dependent Scaling Factor for Treselection defined in NPLs 1 and 3 is used, the determination time for cell reselection, T_reselection, can be varied depending on the speed of a mobile station.
Further, in inter-cell handover, the measurement time for measurement reporting (Time To Trigger) can be varied by applying SpeedStateScaleFactors to a mobile station moving at high speed (NPLs 3 and 4).